Microfabricated devices can be used for a variety of fluid applications, including as sensors, valves, pumps and mixers. Microfabricated devices that are intended to operate in fluids must be designed and fabricated to ensure there is adequate fluid sealing of the device, fluid communication between the fluid and the sensing and/or actuation surface of the device, and isolation of the electrical interfaces from the fluid.
Microfabricated devices are often used to conduct chemical sensing or biological assays. For example, a microfabricated sensor can be used to detect the presence or quantify the amount of a biological analyte in a fluid sample. One approach is to immobilize antibodies specific to the analyte of interest on the sensor surface. Typically, an underlying surface chemistry layer is used to facilitate binding of biological molecules (e.g., antibodies) to a surface of the sensor. Analyte in a fluid sample binds to the immobilized antibodies on the sensor producing a change in the sensor response.
Microfabricated devices also can be used to characterize fluid properties. For example, microfabricated devices are sometimes used to measured density, viscosity and speed of sound in a fluid. Further, by monitoring the response of a sensor to changes in the fluid environment, various chemical or biochemical processes (e.g., reaction rate, phase change, aggregation) can be characterized. Some applications include, but are not limited to, monitoring polymerization processes or measuring material melt curves.
Some sensing applications are performed in conductive fluids (e.g., fluids that contain dissolved salts). Contact between a conductive fluid and electrical traces on the surface of the microfabricated device can result in electrical dissipation into the fluid or changes to the electrical impedance between adjacent electrical traces. Dissipation into a conductive fluid results in a loss of performance and potentially, heating of the microfabricated device and the fluid. A change in the electrical impedance between adjacent electrical traces also interferes with the proper functioning of the microfabricated device in cases where it is necessary to have high impedance isolation between adjacent electrical traces. Further, microfabricated devices are sometimes used in fluids that are corrosive or are capable of adversely affecting the operation of the microfabricated device.
Some microfabricated devices (e.g., traditional flexural plate wave microfabricated devices) have a suspended membrane that is produced by performing a through-wafer etch. The electrical interface to the microfabricated devices resides on the side of the wafer opposite to the etched cavity. In operation, a fluid contacts the sensing surface of the microfabricated device through the cavity formed in the wafer. While this allows the fluid and electrical interfaces to be separated (located on opposite sides of the microfabricated device), it complicates the design of a fluid system using the microfabricated device and can result in poor performance. As the lateral dimensions of the device are reduced, injecting the fluid into the cavity becomes more challenging.
For example, some assays require uniform flow properties at the sensing surface (e.g., a membrane) of the microfabricated device. Injecting fluid into an etched cavity that defines the sensing surface results in a non-uniform fluid flow pattern at the sensing surface. In these cases, it may be advantageous to design a system that provides a substantially planar surface over which the fluid is directed.
In addition, chemically treating the sensing surface of the microfabricated device is challenging because the chemical treatment (e.g., a fluid) must be able to completely wet the cavity. Proper wetting of the cavity is difficult because the cavity has sharp corners (e.g., produced by, for example, a deep reactive ion etching process) which makes it difficult to insure that the entire cavity is adequately treated. Additionally, subsequent use of the device in operation is also challenging because the fluid introduced into the cavity of the microfabricated device also requires proper wetting of the cavity.
A need therefore exists for improved designs for microfabricated devices and improved methods for fabricated microfabricated devices.